1. Field of the Invention
The present invention relates to a slice level deciding circuit in an apparatus serving to read out a picture image.
2. Description of the Prior Art
FIGS. 1A and 1B of the accompanying drawings illustrate a prior slice level deciding system. Operation of the system will be described below with reference to FIGS. 2A-2E.
A surface of a readout medium 1 having characters, symbols and patterns written thereon is scanned by a light beam from a light source 2 to illuminate the surface to provide a line thereon. FIG. 2(A) illustrates an example of the readout medium 1. Dotted lines show ruled lines written with a dropout color, and symbols: 1, 2, T, --, A, B, and C, are examples of picture image information to be read out. In FIGS. 1(A) and 1(B), the readout medium 1 is scanned from the upper end to the lower end (in a direction of arrows Y.sub.1, Y.sub.2) by a light beam from the light source 2. A picture image light beam reflected from the readout medium 1 is transmitted along a light beam propagation path 3, adjusted to be uniform in its quantity by a shading plate 4, and projected onto a photodetector 6 via a lens 5. The reflected light projected onto the photodetector 6 is converted into an electric signal as a picture image signal, which is then amplified by an amplifier 7 and inputted into a comparator 10 as voltage V. By contrast, voltage generated by a reference voltage generator 8 is adjusted by means of a volume control 9 to provide reference voltage, and then inputted into the comparator 10. The comparator 10 serves to compare the voltage V of the image signal with the reference voltage Vo. Namely, if V&gt;Vo, a picture image corresponding to the voltage V is decided to be white-colored, and if V.ltoreq.Vo, a picture image decided black-colored, as shown in FIGS. 2(C) through (E). FIG. 2(B) shows an ideal waveform of the voltage V=Vb corresponding to a scanning light beam line provided on the readout medium when no picture image is written on the readout medium 1. In this instance, also the comparator 10 outputs a signal voltage corresponding to a picture image being white (voltage Va) over the whole area of the readout medium. FIG. 2(C) shows an actual waveform of the voltage V=Vc corresponding to a light beam line on the readout medium as a picture image when no pattern is written on the readout medium 1. The upper part of the waveform waves due to shading adjustment. The figure shows that ruled lines written with a dropout color on the readout medium 1 are outputted. In addition, the comparator 10 outputs a signal of a picture image being white (voltage Va) over the whole area thereof. FIG. 2(D) shows a waveform of the voltage V=Vd of a light beam line provided on the readout medium as a picture image, and the comparator 10 outputs a signal representative of a picture image being black (voltage 0) for a fraction of the voltage V of the picture image signal lower than the reference voltage Vo. FIG. 2(E) shows a waveform of the voltage V=Ve of the picture image signal when an average level of the signal is lowered due to the influence of changes in the quantity of light from the light source due to changes of ambient temperature and the light source, as well as deterioration with age of the readout medium 1 itself. In this instance, ruled lines written on the readout medium 1 with a dropout color are also outputted as black-colored (Voltage 0).
Exemplifying the elements described above, the light source 2 is an ordinary fluorescent lamp; the shading plate 4 is made of a metallic plate formed into a prescribed shape; the photodetector 6 is available from Toshiba as TCD 102C; the amplifier 7 is available from National Semiconductor as LF357N; the reference voltage generator comprises NEC-made RD5. 1E-B2; and the comparator is available from National Semiconductor as LM319N.
As described above, according to a prior slice level deciding system, a picture image signal from the readout medium is converted from a light signal to an electric signal by a CCD type line sensor, and then a slice level used for deciding whether the picture image signal is white or black is set to a desired valve upon adjusting the system. Accordingly, changes in the quantity of light from the light source used for obtaining a picture image signal from the readout medium due to ambient temperature changes around the light source, and changes in a picture image signal yielded from portions on the readout medium where the quantity of light is partly reduced are displayed as noise.